Bill Bill - 5 months ago 37
Perl Question

In Perl, is there a built in way to compare two arrays for equality?

I have two arrays of strings that I would like to compare for equality:

my @array1 = ("part1", "part2", "part3", "part4");
my @array2 = ("part1", "PART2", "part3", "part4");


Is there a built-in way to compare arrays like there is for scalars?
I tried:

if (@array1 == @array2) {...}


but it just evaluated each array in scalar context, and so compared the length of each array.

I can roll my own function to do it, but it seems like such a low-level operation that there should be a built-in way to do it. Is there?

Edit: sadly, I don't have access to 5.10+ or optional components.

Answer

There is the new smart match operator:

#!/usr/bin/perl

use 5.010;
use strict;
use warnings;

my @x = (1, 2, 3);
my @y = qw(1 2 3);

say "[@x] and [@y] match" if @x ~~ @y;

Regarding Array::Compare:

Internally the comparator compares the two arrays by using join to turn both arrays into strings and comparing the strings using eq.

I guess that is a valid method, but so long as we are using string comparisons, I would much rather use something like:

#!/usr/bin/perl

use strict;
use warnings;

use List::AllUtils qw( each_arrayref );

my @x = qw(1 2 3);
my @y = (1, 2, 3);

print "[@x] and [@y] match\n" if elementwise_eq( \(@x, @y) );

sub elementwise_eq {
    my ($xref, $yref) = @_;
    return unless  @$xref == @$yref;

    my $it = each_arrayref($xref, $yref);
    while ( my ($x, $y) = $it->() ) {
        return unless $x eq $y;
    }
    return 1;
}

If the arrays you are comparing are large, joining them is going to do a lot of work and consume a lot of memory than just comparing each element one by one.

Update: Of course, one should test such statements. Simple benchmarks:

#!/usr/bin/perl

use strict;
use warnings;

use Array::Compare;
use Benchmark qw( cmpthese );
use List::AllUtils qw( each_arrayref );

my @x = 1 .. 1_000;
my @y = map { "$_" } 1 .. 1_000;

my $comp = Array::Compare->new;

cmpthese -5, {
    iterator => sub { my $r = elementwise_eq(\(@x, @y)) },
    array_comp => sub { my $r = $comp->compare(\(@x, @y)) },
};

This is the worst case scenario where elementwise_eq has to go through each and every element in both arrays 1_000 times and it shows:

             Rate   iterator array_comp
iterator    246/s         --       -75%
array_comp 1002/s       308%         --

On the other hand, the best case scenario is:

my @x = map { rand } 1 .. 1_000;
my @y = map { rand } 1 .. 1_000;
              Rate array_comp   iterator
array_comp   919/s         --       -98%
iterator   52600/s      5622%         --

iterator performance drops quite quickly, however:

my @x = 1 .. 20, map { rand } 1 .. 1_000;
my @y = 1 .. 20, map { rand } 1 .. 1_000;
              Rate   iterator array_comp
iterator   10014/s         --       -23%
array_comp 13071/s        31%         --

I did not look at memory utilization.